1. Field of the Invention
The present invention is related to a switching regulator having means capable of increasing the efficiency of the switching regulator operated under a light load.
2. Description of the Related Art
As one typical switching regulator, a control circuit of such a conventional switching regulator as shown in FIG. 10 is known. In this conventional switching regulator, a reference voltage circuit 10, bleeder resistors 11 and 12, and an error amplifier 13 are provided. The bleeder resistors 11 and 12 divide an output voltage Vout of the switching regulator. The error amplifier 13 amplifies a difference voltage between the reference voltage of the reference voltage circuit 10 and a voltage appearing at a junction point between the bleeder resistors 11 and 12. Assuming that the output voltage of the error amplifier 13 is Verr, the output voltage of the reference voltage circuit 10 is Vref, and the voltage appearing at the junction point between the bleeder resistors 11 and 12 is Va, if Vref&gt;Va, then the output voltage Verr of the error amplifier 13 becomes high. Conversely, if Vref&lt;Va, then this output voltage Verr becomes low.
A PWM (Pulse Width Modulation) comparator 15 compares the output of an oscillator circuit 14, for instance, a triangular wave, with the output of the error amplifier 13 to thereby output a signal. These signals are illustrated in awaveform chart of FIG. 11. In other words, since the output voltage Verr of the error amplifier 13 is increased/decreased, the width of the output pulse from the PWM comparator 15 is controlled. The switching regulator controls the switch elements with be turned ON/OFF during the time duration of this pulse width. This operation is referred to as a so-called "PWM operation" of the switching regulator.
In FIG. 12, there is shown a structural diagram of a booster type switching regulator. A coil 21 is connected to an input power supply 20. A rectifier element 23 is connected between the coil 21 and an output capacitor 24. A load 25 is connected in parallel to the output capacitor 24. A switching regulator control circuit 30 is connected to an output terminal of the switching regulator so as to turn ON/OFF a switch element 22 of the switching regulator.
In general, in a case where the turn-ON time of a switch element in a switching regulator is prolonged, a power supply capability to a load is increased. For example, when the load becomes heavy, namely when the output load current value is increased, the output voltage of the switching regulator is lowered, and then the voltage Va subdivided by the bleeder resistors 11 and 12 is lowered. As a result, the output voltage Verr of the error amplifier 13 is increased, so that the pulse width of the PWM comparator 15 is widened, and the pulse width is controlled in such a manner that the output voltage Vout is kept constant.
Conversely, when the load becomes light, namely when the output load current value is decreased, the output voltage of the switching regulator is increased, and then the voltage Va subdivided by the bleeder resistors 11 and 12 is increased. As a result, the output voltage Verr of the error amplifier 13 is decreased, so that the pulse width of the PWM comparator 15 is narrow, and the pulse width is controlled in such a manner that the output voltage Vout is kept constant.
That is to say, the output voltage Verr of the error amplifier 13 is varied in response to the load current value, so that the pulse width of the switching regulator is controlled.
However, in the conventional switching regulator, the efficiency of the switching regulator operated under a light load is considerably lowered.